We propose to continue our measurements of RNA sequence complexity in various rat tissues and tumors by RNA driven hybridization to iodinated unique sequence DNA. (1) We hope to determine how sequence complexity of neural tissues changes during development by assaying the RNA complexity of embryonic and newborn brain tissues. (2) We are also planning to assay the RNA sequence complexity of clonal neural tumor lines. Using these clonal lines we can assay the same cells either in tissue culture or as passaged in vivo as a solid tumor. We are particularly interested in the nuclear RNA complexity of the clonal lines, since the cell line cytoplasmic RNA complexity is similar to non-neural tissue being about one-fourth that of the total brain. (3) To accurately estimate how many RNA species are actually translated into protein we will determine the RNA sequence diversity of puromycin released polysomal RNA. Our purification procedures should eliminate any nuclear RNA which might contaminate cytoplasmic RNAs.